Transmission of dominant transverse electric mode in large rectangular waveguide, with polarization parallel to width, by use of mode absorber



I. KHOURY 3,218,536 TRANSMISSION OF DOMINANT TRANSVERSE ELECTRIC MODE IN LARGE Nov. 16, 1965 RECTANGULAR WAVEGUIDE, WITH POLARIZATION PARALLEL To WIDTH, BY USE OF MODE ABSORBER Filed April 21, 1961 United States Patent 3,218,586 TRANSMISSION OF DOMINANT TRANSVERSE ELECTRIC MODE IN LARGE RECTANGULAR WAVEGUIDE, WITH POLARIZATION PARALLEL TO WIDTH, BY USE OF MODE ABSORBER Khalil Ibrahim Khoury, London, England, assignor to Decca Limited, a British company Filed Apr. 21, 1961, Ser. No. 104,697 Claims priority, application Great Britain, Apr. 22, 1960, 14,272/60 7 Claims. (Cl. 333-95) This invention relates to waveguide systems for the transmission of microwave signals.

In this specification, the mode of a transverse electric wave will be denoted by the expression TE where the subscripts mi and n refer respectively to the number of half period variations in electric field strength across the shorter and longer sides of the wave guide.

According to this invention, a waveguide system for the transmission of microwave signals of a given frequency comprises a rectangular metal waveguide with means for launching the signals to be propagated into the wavegiude as a plane polarised transverse electric wave with the electric vector parallel to the longer side of the guide, the guide having the dimension of its longer side such that the guide can transmit the signals at said given frequency in more than one mode, and a mode filter in the guide to suppress the transmission of modes in which the electric vector is not parallel to the longer side. The mode filter may comprise a number of resistive films or filaments extending across the guide in planes parallel to the shorter side wall of the guide or a number of slots extending transversely in a longer side wall or walls of the waveguide.

It will be seen that this transmission system employs a rectangular waveguide of larger dimensions than would normally be used and that the signals are transmitted with the electric vector parallel to the longer side of the waveguide whereas, in conventional practice using a rectangular waveguide, the signals are transmitted as a transverse electric wave with the electric vector parallel to the shorter side, the waveguide having dimensions such that only the simplest mode of propagation is transmitted along the guide. By using a very much larger waveguide as described above, provided the signals are propagated only in the simplest mode, the signals can be transmitted with a very much smaller attenuation per unit length than is the case in a rectangular waveguide of conventional size such as would transmit only a single mode and thus it is possible to use very much longer lengths of waveguide transmission systems.

To ensure that only one mode is propagated, considering the variation of electric vector across the shorter side, this dimension may be made sufficiently small to ensure that, modes with more than one half period of variation are evanescent. Preferably, however, the guide has the dimension of this shorter side such that the guide can transmit modes at said given frequency having more than one half period of variation in the electric vector and, to form a mode suppressing filter, the waveguide has a slot or slots in one or each shorter faces of the guide extending in the axial direction to suppress the transmission of modes having more than one half period of variation in the electric vector across the shorter dimension of the waveguide. Instead of a slot or slots, a series of suitably spaced holes may be employed. Most conveniently the guide is made so that, considering the shorter dimension, only the first and second order modes can be propagated and, in that case, a single central axially extending slot or equivalent set of slots or holes in one or each of the shorter sides of the waveguides at the input end will suppress the second order mode. The slot or holes may be provided with a suitable load absorber to prevent radiation into free space.

The invention thus includes within its scope a waveguide system for the transmission of a microwave signal of a given frequency comprising a rectangular metal waveguide of dimensions such as to transmit the signal in a multiplicity of modes with the shorter dimension such that the TE but not the T E mode is evanescent, means for launching the signal to be propagated into the waveguide as a TE wave with the electric vector parallel to the longer side of the guide and mode filter means in the guide comprising a number of resistive films or filaments in planes parallel to the shorter side of the guide or transverse slots in the longer side or sides of the guide to suppress the transmission of all modes except TE modes and a slot or slots or holes extending along the shorter side of the guide and disposed centrally across the width thereof arranged to couple out of the guide any signals in the TE mode.

It is possible, however, to use a Waveguide capable of propagating third or higher order modes, considered in the direction of the shorter side, if provision is made to attenuate these unwanted modes. The above dee scribed central slot or holes would attenuate not only second order modes, but all further even order modes, To suppress a third order mode, it would be necessary, for example, to provide two slots or sets of holes in the narrower face or faces of the waveguide spaced apart across the width of the waveguide and preferably at the nodal points of the third order, the slotsvor holes connecting to adjacent guides or cavities to absorb the third order mode. Several such pairs of slots or sets of holes,- for example, may be arranged along the waveguide to act as coupling holes to an auxiliary waveguide, the coupler so formed being designed to give high coupling to the unwanted third order mode and low coupling to the wanted first order mode. If the slots are coupled to a cavity, the coupling may be arranged so that the third order mode would set up a resonance in the cavity so that the latter absorbs energy from the third order mode in the main waveguide but not from the first order mode.

At the far end of the waveguide, it will in general be necessary to provide a transition section coupling the larger rectangular waveguide to a waveguide of conven-' tional dimensions, for example a smaller section rectangular waveguide capable of propagating only the simplest mode, and in that case it is preferable to pro vide a mode filter not only at the input end of the large dimension waveguide but also at the output end so as to prevent reflections of unwanted modes from any transitions at the output. Each mode filter may. conveniently comprise a length of rectangular waveguide, which may be a few wavelengths long, having a series of resistive films parallel to the short side of the wave-, guide and slots or sets of holes in the shorter side of the waveguide as described above. This mode filter is most conveniently made as a waveguide of the same dimensions as the large dimension waveguide.

The resistive films may conveniently comprise evaporated metal films, for example evaporated nickel-chromium alloy films, on thin glass or mica sheets which may be secured in the waveguide in parallel planes by any suitable means.

At each end, it may be required to connect the transmission system to a smaller section rectangular wave guide capable of propagating only a single mode and, for this purpose, conveniently straight taper transition sections are employed; for dimensions such as might be used in practice, in general the taper sections would have to be made as long as possible to minimise losses due to mode coupling in the transition. Since, in practice, the manufacturing tolerances have to be greater in a long taper section and the absorption losses will increase with increase of length of the taper section, the length of the taper section would be chosen to keep the overall losses to a minimum.

In the following description reference will be made to the accompanying drawings in which:

FIGURE 1 is a diagram illustrating a transmission system for microwave signals;

FIGURES 2 and 3 are diagrams illustrating the sections along the lines 22 and 33 respectively of the waveguide employed in FIGURE 1;

FIGURES 4 and 5 are respectively an end view and a sectional plan of a mode filter used in the arrangement of FIGURE 1;

FIGURES 6 and 7 are respectively an end view and a sectional plan of another form of mode filter;

FIGURE 8 is a perspective view illustrating a right angle bend in the waveguide system of FIGURE 1; and

FIGURES 9 and 10 are respectively a longitudinal section and an end view of a pressure window element in the waveguide of FIGURE .1.

FIGURE 1 illustrates diagrammatically a transmission system for transmitting microwave signals from a source 10 to a load 11. The source 10 is coupled to a rectangular metal waveguide 12 of conventional dimensions so that the signals from the source are propagated through the waveguide 12 as a TE wave with the electric vector parallel to the shorter sideof the waveguide. By a waveguide of conventional size it is to be understood that this waveguide 12 has dimensions such that it can only support propagation of a TE wave, all higher order modes being evanescent.

As is well known, a waveguide of conventional size propagating a TE modev will transmit signals over a distance but there is appreciable attenuation which puts a limit on the length of waveguide run which can be employed in practice. In order to reduce this attenuation and so to enable a longer length of waveguide to be employed, in the arrangement of the present invention the signals from the waveguide 12 are fed into a metal waveguide 13 of rectangular section having dimensions substantially larger than the waveguide 12. The waveguide 13 in the particular embodiment shown in FIGURE 1 has dimensions such that it would support a multiplicity of modes of propagation, the shorter dimension being such however that, although a TE mode can be propagated, a TE mode is evanescent. The signal is launched into this waveguide as a TE mode and thus FIGURE 1 shows the narrow dimension of the waveguide 13 but the wider dimension of waveguide 12. Coupling between the waveguide 12 and the waveguide 13 is effected by means of a straight tapered transition section 14 having walls in the form of plane faces which diverge at the appropriate angles to produce a smooth transition between the small section waveguide 12 and the much larger section waveguide 13. This transition section 14 is shown in end View in FIGURE 3 which is a section through the waveguide 13 looking from the larger section waveguide down the transition section to the smaller dimension waveguide 12.

For dimensions such as are used in practice, to minimise losses due to mode coupling in the transition section 14, this taper section would have to be made as long as possible. However, beyond a certain length no further improvement if obtained because of mechanical inaccuracies and, in any case, the improvement due to reduced coupling to higher order modes is offset by increased attenuation because of the small cross-section. The length of the transition section 14 is therefore chosen to keep to a minimum the overall losses due to attenuation and to higher order mode coupling.

The large section waveguide 13 may include bends and, in FIGURE 1, two right angle bends are illustrated at 15 as an example of such bends. At the far end of the waveguide 13 a further transition section 16 is provided which is similar to the transition section 14 at the input; end and provides a straight taper leading from the large section waveguide 13 to a small rectangular waveguide 17, which will typically be of the same dimensions as the waveguide 12 and in which the signals are propagated at a TE wave for feeding into the load 11.

Although the signals are launched into the large section Waveguide 13 as a TE mode, due to resonances of the unwanted modes between the two ends of the waveguide, energy from the required TE mode may be absorbed at the resonant frequencies. With a long length of waveguide run such resonances will occur at a large number of frequencies which may be quite closely spaced in the frequency band at which the transmission system is to be employed. In general it will not be possible to choose the signal frequency such as to avoid these resonances and, in the arrangement of the present invention, mode filters are therefore provided to suppress the unwanted modes into which the wanted TE mode may be coupled by these resonances. It would be possible to use one or more such mode filters at any convenient point or points along the transmission path formed by the large section waveguide 13, but very conveniently a mode filter is employed at each end. Such an arrangement is illustrated in FIGURE 1 where there are shown two such mode filters 18. These mode filters are illustrated in further detail in FIGURES 4 and 5. Referring to these figures each mode filter comprises a length of rectangular waveguide of the same section as the waveguide 13, the mode filter being a few wavelengths long. In this guide forming the mode filter, a number of resistive films 20 are spaced apart in planes parallel to the shorter sides of the waveguide 13. These resistive films 20 stretch, the whole way across the waveguide between the two longer side walls and may extend along the length of the mode filter. Conveniently the films 20 are evaporated metal films, for example evaporated nickel-chromium alloy films on thin glass or mica sheets which may be secured in the waveguide by any convenient means. These resistive films 20 will absorb all the modes except the TE modes and in particular will absorb TE modes where n is l, 2 or more. The number of such films would depend on the size of the waveguide and hence on the maximum value 11 of the integer n, the films being sufiiciently close together to ensure that only the TE modes can be propagated. Instead of using resistive films 20, filaments across the waveguide in planes parallel to the narrower faces of the waveguide or transverse slots in the broader faces may be employed.

It is necessary also to suppress the TE mode in the waveguide 13, that is to say a mode in which there are two half periods of variation in the electric vector across the waveguide considered in the narrow dimension thereof. For this purpose there are provided a series of slots 21 arranged centrally along the narrower faces of the waveguide in each of the two mode suppression filters 18. Typically five or six such slots 21 might be provided on each narrow face of the waveguide. These slots are: arranged to couple the unwanted TE mode so that any signals in this mode pass through the slot out of the waveguide to be absorbed by absorbing material 22 placed around the narrower faces of the waveguide forming the mode suppression filters 18. Because these slots are central, they will not couple out any of the wanted TE mode but they will couple out the TE mode and also any other TE modes where m is an even integer.

The form of mode suppression filter illustrated in FIGURES 4 and 5 can readily be made to have a relatively low loss for the required signals and it has been found that by using an arrangement such as is illustrated in FIGURE 1 with the large waveguide 13 capable of supporting TE modes where m is any integer up to but not exceeding 2 and the maximum value of n is typically substantially greater than 2, it is possible to reduce the attenuation for a given length of waveguide run by a factor of the order of 8 or 10 compared with the use of standard size rectangular waveguide propagating signals in a TE mode.

The attenuation arising in the transmission of signals through a waveguide is due both to absorption of the signals into the walls and to coupling of the wanted mode into unwanted modes due to resonances. The coupling to the unwanted modes is suppressed in the arrangement of FIGURE 1 by the mode suppression filters 18. The absorption of the wanted mode into the walls of the waveguide is reduced compared with that in a system using standard section waveguide propagating TE signals only by using the large section waveguide 13 for the greater part of the length of the transmission path. This latter component of loss in transmission may, for the present purposes, be expressed by the simplified function where a and b are the dimensions of the shorter and longer sides of the waveguide 13 respectively, that is to say the b dimension is a dimension parallel to the electric vector and where C and C are constants. It will be seen that by increasing the longer dimension of the Waveguide 13, that is to say dimension b, the loss is reduced. However, due to the efliect of the first term which is dependent on the dimension (1, there is a practical limit where the term is negligible compared with and beyond which the further reduction in loss due to increase of the dimension [1 therefore becomes negligible. Thus if the mode filter is arranged to suppress TE modes but the waveguide has to have the a dimension sufliciently small that TE modes are not transmitted there is a practical limit to the increase in the b dimension for giving any substantial improvement in performance. Such an arran ement, however, gives a very substantial reduction in loss compared with the loss using a conventional waveguide propagating only TE mode signals and is quite satisfactory for many purposes.

It is possible, however, to use a waveguide with the shorter dimension, i.e. the a dimension, capable of propagating third (TEgo) or higher order modes, considering modes varying in the direction of the shorter side, if provision is made to attenuate the unwanted modes. FIGURES 6 and 7 illustrate a coupler forming a mode filter for coupling out TE modes from a waveguide and having negligible effect on the TE mode. Referring to these FIGURES 6 and 7 there is shown the large section waveguide 13 which in its narrower faces has two parallel rows of slots 25. In FIGURE 6 the electric vector amplitude for a TE mode is indicated by a graphical curve 26 in dash lines, this representing, with respect to a base line 27, the distribution of amplitude across the waveguide. Similarly the electric vector amplitude for a T E mode is represented by a dash line curve 28 with respect to a base line 29. The slots are arranged at the nodal points for the TE mode and would have only a small coupling to the TE mode. However, the signals fed out through the slots pass into adjoining waveguides or cavities 30 (FIGURE 6). If adjoining waveguides are employed, the pairs of slots are coupled to the adjoining waveguide to give high coupling for the third order mode but low coupling for the first order mode so that, in the adjoining waveguides 30, the third order mode only is absorbed. If the slots are coupled to cavities, the coupling is arranged so that the third order mode would set up 6 a resonance in the cavity so that the latter absorbs energy from the third order mode in the main waveguide but not from the first order mode. It will be understood that the mode suppression filter illustrated in FIGURES 6 and 7 for suppressing the TE mode would have to be provided in addition to the mode suppression filters illustrated in FIGURES 4 and 5 for coupling out the second and higher even order modes. The mode suppression filter of FIGURES 4 and 5 would not have any efiect on the odd order modes.

It is commonly necessary to have bends in a waveguide run such as the bends 15 in FIGURE 1. For the purpose of the transmission system embodying the large dimension rectangular waveguide, it is generally desirable to avoid at such bends any possibility of coupling of the wanted TE mode into an unwanted mode and a construction of bend for this purpose is illustrated in FIGURE 8. Referring to that figure it will be seen that there are two straight portions of the rectangular metal waveguide 13 which are joined by an H plane bend 35, that is to say a bend in the longitudinal plane transverse to the direction of the electric vector. The bend 35 is formed of rectangular section metal waveguide similar to the waveguide 13 but at its two ends where it joins the straight sections of waveguide the bend is displaced in the direction of the shorter side of the waveguide. The amount of displacement is dependent on the radius of the bend. The resultant shoulders 36 in the waveguide run are closed with conductive material. If the bend were not displaced there would be a certain amount of coupling between the TE mode in the straight sections of waveguide and the TE mode in the bend and this coupling can be minimised by displacing the bend relative to the straight sections in the direction towards the centre of curvature as shown in FIGURE 8 so that effectively the TE mode in the straight sections is coupled only to the TE mode in the bend.

It would be possible to avoid coupling into unwanted modes at a bend in other ways, for example by changing the dimensions of the waveguide at the bend or by using irises or by using a suitable dielectric material in the waveguide.

It may be necessary to pressurise the waveguide 13 with dry air or at least to ensure that the air contained therein is dry air. For this purpose the waveguide has to be sealed in an airtight manner and very conveniently the slots employed in the mode suppression filters may be used for introducing the dry or pressurised air into the waveguide. In such an arrangement the end of the waveguide has to be sealed and for this purpose the form of window illustrated in FIGURES 9 and 10 may be used. Referring to these figures there is shown a block of matched dielectric material 40 elfectively half a wavelength long inserted in a length of rectangular waveguide 41. Two such windows conveniently are arranged one at each end of the waveguide run 13 or in the mode suppression filters 18. The rectangular waveguide 41 is of the dimensions of the waveguide 13 and has an inductive non-resonant iris formed by projections 42 extending inwardly from the longer sides of the waveguide wall, these projections extending for the full height of these sides and forming shoulders to support the dielectric material 40 so that the latter can withstand an air pressure on one end face. By providing a thick iris it can be arranged that there is very little mismatch to the wanted mode of propagation.

I claim:

1. A waveguide system for the transmission of a microwave signal of a given frequency comprising a rectangular metal waveguide of dimensions such as to transmit the signal in a multiplicity of modes but with the shorter dimension such that the TEgg but not the TE mode is evanescent, means for launching the signal to be propagated into the waveguide as a T E wave with the electric vector parallel to the longer side of the guide and mode filter means in the guide comprising a number of resistive elements in planes parallel to the shorter side of the guide to suppress the transmission of all modes except TE modes and at least one slot extending along the shorter side of the guide and disposed centrally across the width thereof arranged to couple out of the guide any signals in the TE mode.

2. A waveguide system for the transmission of a microwave signal of a given frequency comprising a rectangular metal waveguide of dimensions such as to transmit the signal in a multiplicity of modes but with the shorter dimension such that the TE but not the TE mode is evanescent, means for launching the signal to be propagated into the waveguide as a TE wave with the electric vector parallel to the longer side of the guide and mode filter means in the guide comprising transverse slots in at least one of the longer sides of the guide to suppress the transmission of all modes except TE modes and at least one slot extending along the shorter side of the guide and disposed centrally across the width thereof arranged to couple out of the guide any signals in the TE mode.

3. A waveguide system for the transmission of microwave signals of a given frequency comprising a rectangular metal waveguide with means for launching the signals to be propagated into the waveguide as a plane polarized wave with the electric vector parallel to the longer side of the guide, the guide having the dimensions of its longer side such that the guide can transmit the signals at said given frequency in more than one mode, and a mode filter in the guide comprising a number of resistive films in planes parallel to the shorter side wall of the guide to suppress the transmission of modes in which the electric vector is not parallel to the longer side, the guide having dimensions of the shorter side such that the guide can transmit modes at said given frequency having more than one half period of variation in the electric vector and having a mode suppression filter comprising at least one slot in at least one of the shorter faces of the guide along the axial direction thereof to suppress the transmission of modes having more than one half period of variation in the electric vector across the shorter dimension of the waveguide.

4. A waveguide system as claimed in claim 3 wherein the waveguide is made so that, considering the shorter dimension, only the first and second order modes can be propagated and wherein said mode suppression filter for preventing the transmission of the second order mode comprises a central axially extending slot in at least one of the shorter sides of waveguides.

5. A waveguide system as claimed in claim 3 wherein the waveguide is made so that, considering the shorter dimension, the fifth order mode cannot be propagated and wherein, to suppress the third order mode, there are provided at least two slots in the narrower face or faces of the waveguide spaced apart cross the Width of the waveguide and connecting to adjacent guides to absorb the third order mode.

6. A waveguide system as claimed in claim 5 wherein said slots are spaced apart to be at the nodal points of the third order mode.

7. A waveguide system for the transmission of microwave signals of a given frequency comprising a rectangular metal Waveguide with means for launching the signals to be propagated into the waveguide as a plane polarized wave with the electric vector parallel to the longer side of the guide, the guide having the dimensions of its longer side such that the guide can transmit the signals at said given frequency in more than one mode, and a mode filter in the guide comprising a number of resistive films in planes parallel to the shorter side wall of the guide to suppress the transmission of modes in which the electric vector is not parallel to the longer side, the guide having dimensions of the shorter side such that the guide can transmit modes at said given frequency having more than one half period of variation in the electric vector and wherein a mode suppression filter is provided comprising a series of suitably spaced holes in at least one of the shorter faces of the guide along the axial direction thereof to suppress the transmission of modes having more than one half period of variation in the electric vector across the shorter dimension of the waveguide.

References Cited by the Examiner UNITED STATES PATENTS 2,476,034 7/1949 Fox.

2,673,962 3/1954 Kock 333-98 2,684,469 7/1954 Sensiper 333-98 2,735,069 2/1956 Riblet 333-81 X 2,764,743 9/1956 Robertson 333-98 2,869,085 1/1959 Pritchard et al 333-98 2,958,834 11/1960 Symons et al. 333-98 2,981,906 4/1961 Turner 333-98 2,981,907 4/1961 Bundy 333-81 3,076,188 1/1963 Schneider 333-21 X 3,078,423 2/1963 Lewis 333-73 X FOREIGN PATENTS 671,206 4/ 1952 Great Britain.

HERMAN KARL SAALBACH, Primary Examiner.

BENNETT G. MILLER, ELI LIEBERMAN, Examiners. 

3. A WAVEGUIDE SYSTEM FOR THE TRANSMISSION OF MICROWAVE SIGNALS OF A GIVEN FREQUENCY COMPRISING A RECTANGULAR METAL WAVEGUIDE WITH MEANS FOR LAUNCHING THE SIGNALS TO BE PROPAGATED IN TO WAVEGUIDE AS A PLANE POLARIZED WAVE WITH THE ELECTRIC VECTOR PARALLEL TO THE LONGER SIDE OF THE GUIDE, THE GUIDE HAVING THE DIMENSIONS OF ITS LONGER SIDE SUCH THAT THE GUIDE CAN TRANSMIT THE SIGNALS AT SAID GIVEN FREQUENCY IN MORE THAN ONE MODE, AND A MODE FILTER IN THE GUIDE COMPRISING A NUMBER OF RESISTIVE FILMS IN PLANES PARALLEL TO THE SHORTER SIDE WALL OF THE GUIDE TO SUPPRESS THE TRANSMISSION OF MODES IN WHICH THE ELECTRIC VECTOR IS NOT PARALLEL TO THE LONGER SICE, THE GUIDE HAVING DIMENSIONS OF THE SHORTER SIDE SUCH THAT THE GUIDE CAN TRANSMIT MODES AT SAID GIVEN FREQUENCY HAVING MORE THAN ONE HALF PERIOD OF VARIATION IN THE ELECTRIC VECTOR AND HAVING A MODE SUPPRESSION FILTER COMPRISING AT LEAST ONE SLOT IN AT LEAST ONE OF THE SHORTER FACES OF THE GUIDE ALONG THE AXIAL DIRECTION THEREOF TO SUPPRESS THE TRANSMISSION OF MODES HAVING MORE THAN ONE HALF PERIOD OF VARIATION IN THE ELECTRIC VECTOR ACROSS THE SHORTER DIMENSION OF THE WAVEGUIDE. 